1. Field of the Invention
The present invention relates to a lens accessory mounting device for mounting a lens accessory such as a lens hood, filter, etc., to a lens barrel of a camera.
2. Description of the Related Art
Screw structures and bayonet structures have been conventionally used as devices for mounting a lens accessory such as a lens hood, a filter (optical filter), etc., to the front end of a lens barrel of a camera. The screw structure is composed of a female screw thread portion formed on an inner peripheral surface of a lens barrel at the front end thereof, and a male screw thread portion formed on an outer peripheral surface of the mounting portion of a lens accessory such as a lens hood or a filter so that the male screw thread portion is screw-engaged with the female screw thread portion when the lens accessory is mounted to the lens barrel. In such a screw structure, it is difficult to mount the lens accessory to the lens barrel with the lens accessory being positioned at a specified angle of rotation relative to the lens barrel about the optical axis thereof, and accordingly, the screw structure is used for a lens hood or a filter which is not restricted to such a specified angle.
On the other hand, the bayonet structure makes it possible to mount the lens accessory to the lens barrel with the lens accessory being positioned at a specified angle of rotation relative to the lens barrel about the optical axis thereof, and accordingly, the bayonet structure is used for a petal lens hood or a lens hood having a non-circular-shaped front aperture. As disclosed in Japanese unexamined patent publication H06-265767, the bayonet structure is composed of a set of bayonet lugs or bayonets formed on an outer peripheral surface of a lens barrel at the front end thereof, and another set of bayonet lugs or bayonets formed on an inner peripheral surface of a lens hood at the rear end thereof, wherein firstly the rear end of the lens hood and the front end of the lens barrel are engaged with each other in the optical axis direction of the lens barrel without the set of bayonet lugs of the lens barrel and the set of bayonet lugs of the lens hood overlapping one another in the optical axis direction of the lens barrel, and subsequently, the lens hood and the lens barrel are rotated relative to each other about the optical axis of the lens barrel by a small amount of rotation so that the set of bayonet lugs of the lens barrel are engaged with the set of bayonet lugs of the lens hood, respectively. Each set of bayonet lugs are provided as lugs arranged at different circumferential positions, and the relative position of engagement between the two sets of bayonet lugs can be specified by differentiating the intervals or the lengths of each set of bayonet lugs in a circumferential direction from one another so that the lens hood can be mounted and fixed to the lens barrel at a desired angle of rotation relative to the lens barrel about the optical axis thereof.
As shown in the bayonet structure disclosed in the aforementioned patent publication, a conventional bayonet structure is provided with a set of bayonet lugs formed on an outer peripheral surface of a lens barrel at the front end thereof, and another set of bayonet lugs formed on an inner peripheral surface of the mounting portion of a lens hood. Therefore, in a state where no lens hood is mounted to the lens barrel, the bayonet lugs on an outer peripheral surface of the lens barrel at the front end thereof are exposed, which deteriorates the outward appearance of the lens barrel or the camera to which the lens barrel is mounted. Additionally, the structure of the bayonet lugs which project radially outwards from an outer peripheral surface of the lens barrel at the front end thereof limits the lens design at the front end of the lens barrel, and especially makes the design for mounting a lens cap to the front end of the lens barrel difficult. Specifically, in small and slim cameras produced in recent years, slim lens hoods having a non-circular aperture (usually a rectangular aperture corresponding to the shape of a rectangular picture plane) are available on the market. However, in the case where this type of lens hood is mounted to the lens barrel with a bayonet structure, the outer diameter of the front end of the lens barrel (to which the lens hood is mounted) increases due to the lens hood, which makes it impossible to attach a normal lens cap, which is designed to be attached to the front end of the lens barrel, to the front of the lens barrel (i.e., the front end of the lens hood). In this case, when it is desired to attach a lens cap to the front of the lens barrel, it is necessary to prepare an extra lens cap which has a larger diameter than the normal lens cap in order to correspond to the outer diameter of the lens hood, which increases the number of parts of the camera system.